This invention relates to a hydrostatic winch, and more particularly, to a hydrostatic winch having a hydraulic control system with a dual valve which controls the hoisting and lowering speed of the winch and a brake for stopping the load, if necessary.
Hydrostatic winches for hoisting or lowering loads operate most efficiently at relatively high speeds and relatively constant horsepower. However, winches run at speeds near their upper limits can enter into an overspeed condition, especially when lowering a load. An overspeed condition should be quickly identified and corrected to prevent damage to the winch system.
Several control systems have been proposed to prevent overspeed damage. The control system described in U.S. Pat. No. 3,943,713 issued to Erlen Walton for a "Control Arrangement" compares the actual speed of the winch motor to a desired speed as defined by a reference speed motor using a gear arrangement, and adjusts the displacement of the pump and motor of the winch drive accordingly. The Walton winch also includes a braking system for quickly bringing the load to an abrupt stop in the event that an abnormal condition occurs such as a broken hydraulic line or high pump or motor case drain pressure. U.S. Pat. No. 3,685,290 issued to Alfred Krusche for an "Overload System for a Hydrostatic-Drive Apparatus" also discloses a braking system which brings the load to an abrupt stop.
The hydrostatic winch of the present invention includes not only an improved overspeed control system, but also an improved fail safe system for smoothly stopping the load to eliminate the strain placed on the winch system by abrupt braking of the load.